Polarization Mode Dispersion (PMD) occurs in an optical fiber as a result of a small residual birefringence that is introduced in the fiber core by asymmetric internal stress as well as random polarization coupling due to external forces acting upon the fiber. It is well-known that PMD may severely impair the transmission of a signal in an optical fiber network. This is especially the case in modern digital lightwave systems that operate at bit rates of at least 10 Gb/s per transmitted wavelength channel.
U.S. Pat. No. 5,930,414, which issued Jul. 27, 1999 to D. A. Fishman et al, discloses different apparatus for mitigating signal impairments due to PMD. Such apparatus employs a variable optical birefringence element which introduces a differential optical time delay between at least two selectable mutually orthogonal polarization states, i.e., the "Principal States of Polarization" (PSP) in the transmission fiber. An optical distortion analyzer coupled to the output of the variable birefringence element generates a control signal for that element.
The compensation apparatus shown in FIG. 4 of the referenced '414 patent generates a continuously variable Differential Group Delay (DGD) between two principal states of polarization to compensate for first-order PMD in the transmission fiber. One difference between this arrangement and the time-delay line shown in FIG. 1 of the referenced '414 patent is that the PSPs of the variable birefringence element in the apparatus of FIG. 4 of the '414 patent is frequency dependent. Such frequency dependence (which is an important aspect of a so-called second-order PMD effect) may lead to excessive distortion in the optical signal, since the first-order PMD in the fiber can only be compensated for over a certain optical bandwidth. Moreover, the PMD may even increase in certain parts of the optical spectrum.
Signal distortion caused by the second-order PMD effect in conventional non-return-to-zero (NRZ) and return-to-zero (RZ) digital optical signals may be observed in the electrical spectrum of the received optical signal as a narrowband "tone" of spectral energy at the certain frequencies related to the signal bit rate, e.g., 10 GHz for NRZ and 20 GHz for RZ signals.